degree Celsius

SI coherent derived unit with special name and symbol

Name

Symbol

Derived quantity

Expressed in terms of SI base units

degree Celsius

°C

Celsius
temperature

Definition

The unit of Celsius temperature is the degree Celsius, symbol °C. It is an SI derived unit of temperature. The degree Celsius is by definition equal in magnitude to the unit kelvin.

The degree Celsius can also be used to express a specific temperature on the Celsius scale. Zero degrees on the Celsius scale (0 °C) is defined as 273.15 K.

The degree Celsius is named after the Swedish astronomer Anders Celsius (1701 – 1744).

A difference in temperature may be expressed in kelvins or in degrees Celsius, the numerical value of the temperature difference being the same in either case:

$\Delta t / ^{\circ} \text{C} = \Delta T / \text{K}$

The Celsius scale

For historical reasons, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T₀ = 273.15 K, close to the ice point.

$t = T - T_0$

where:

t is the Celsius temperature in °C,
T is the thermodynamic temperature in K,
T₀ is the thermodynamic temperature in K, equal to 0 °C.

When used to express a specific temperature on the Celsius scale, the numerical value of a Celsius temperature is related to the numerical value of the thermodynamic temperature expressed in kelvins by the relation:

$t / ^{\circ} \text{C} = T / \text{K} - 273.15$

Examples of everyday use

Usage	Temperature range
outdoor thermometer	-50 – 50 °C
central heating thermostat	5 – 30 °C
clinical thermometer ^*	35 – 42 °C
domestic oven	50 – 250 °C
refrigerator	1 – 5 °C
freezer	-25 – -18 °C

^* Normal body temperature = 37 °C.

Usage

Note: The symbol for degree Celsius should always include the ° sign.
i.e. °C, and never just C.

History

The Celsius scale is a decimal temperature scale that was created prior to the development of the metric system. Its original definition was based on the properties of water.

In 1742, Anders Celsius created a temperature scale that was the reverse of the scale known today as the Celsius scale: zero represented the boiling point of water, and 100 represented the freezing point of water. He proposed that the zero point of his temperature scale would be calibrated at the average pressure at mean sea level. This pressure was later defined as 101.325 kPa.

In 1743, the physicist Jean-Pierre Christin developed a scale where zero represented the freezing point of water, and 100 represented the boiling point of water.

In 1948, the temperature scale, which had hitherto been called “centigrade”, was renamed Celsius, in honour of Anders Celsius.

In 1954, the Celsius scale, and the degree Celsius, were defined by absolute zero and the triple point of a precisely defined water standard known as Vienna Standard Mean Ocean Water, VSMOW. The triple point of water was set to be exactly 273.16 K.

In 2019, the kelvin and the degree Celsius were redefined by fixing the numerical value of the Boltzmann constant, k.